Mostly foreign fighters of al-Qaeda affiliated Jabhat al-Nusra and ISIS (Islamic State of Iraq & Syria) groups are supposed to be fighting against the Assad government, but lately they seem to be spending more time killing each other than anything else. WIN WIN!
ISIS is in the lead but al-Nusra is running a close second. The infighting between the terrorist groups in this area is extreme, because the trophy for the winner is huge. Isis, the old al Qaeda of Iraq and al Nusra, the official al Qaeda representative in Syria including many Obama-backed FSA rebels are fighting over the oil fields of Deir Ez-zor.